1. Field of the Invention
The present invention relates to a white organic light emitting device (WOLED), and more particularly, to a voltage-driven WOLED using electroluminescence and photoluminescence.
2. Description of the Related Art
In general, organic light emitting devices (OLEDs) are spontaneous emission devices. The spontaneous emission devices emit light induced by combinations of electrons and holes. OLEDs have the advantages of higher color reproducibility, faster response speed, smaller thickness, higher contrast ratio, wider viewing angle, and lower power consumption. Such OLEDs may be used in various fields such as display devices and illumination devices.
A white organic light emitting device (WOLED), which is a thin light source using an open mask instead of a high definition mask, is advantageous of higher resolution and larger scale, and can be used in a backlight of a liquid crystal display (LCD) device or a full color display device employing a color filter. In order to increase the luminous efficiency of the WOLED, research has been made on a stacked structure, in which red (R), green (G), and blue (B) light emitting layers are vertically connected in series. Since the light emitting layers require voltages and are connected in series, the voltage applied to the entire stacked structure, that is, a driving voltage, may be very high. When the high driving voltage is applied to the stacked structure, electricity charges of the light emitting layers are disadvantageously forced to be unbalanced to change the distribution of luminous intensity. The changed distribution of luminous intensity may change color coordinates, and thus the desired white color coordinates is difficult to be achieved. Here, the color coordinate is a group of parameters for defining a certain color in a chromaticity diagram, and the parameters may be, for example, hue and saturation.
The main objective of the WOLED is to realize white light having color coordinates (0.31, 0.31). A top emission type WOLED has a structure in which a reflective layer is disposed under an emission structure having a thickness of about 100 nm to about 200 nm, and white light is emitted through an upper portion of the emission structure. An optical path control layer (OPCL) for effectively extracting desired light from a spectrum changed by a cavity is disposed on the emission structure through which the white light is emitted.
Therefore, the major disadvantages of the contemporary WOLED are high driving voltage and the disadvantageous color shift induced by the high driving voltage.